The present invention relates to an intake air control of an internal combustion engine (which will be hereinafter referred to as an “engine”).
Conventionally, a constant negative pressure control for keeping a negative pressure (gauge pressure) in the intake manifold of an engine constant at a desired value by adjusting an opening degree of a throttle valve has been proposed. For example, such a control is disclosed in Japanese Patent Application Publication No. 2000-161120.
According to a conventional scheme for the constant negative pressure control, a table used for determining an opening degree of the throttle valve that implements a desired value of the negative pressure is prepared. When a plurality of desired values of the negative pressure are pre-established, such a table is required for each of the plurality of desired values. One of the tables that corresponds to a current desired value is selected and then an environmental correction process based on the result of a pretest is performed to determine a final value of the opening degree of the throttle valve. Further, when air flows into an intake air system via a route other than the throttle valve (such air is called secondary air), the throttle opening degree is further corrected, for example, by a feedback control of the negative pressure.
According to the conventional scheme for the constant negative pressure control, many process steps are required for determining an opening degree of the throttle valve. A table for determining an opening degree of the throttle valve is required for each of the pre-established desired values of the negative pressure. Thus, a process by one or more software programs for implementing the constant negative pressure control may be complex. Further, depending on the amount of the secondary air, it may be difficult for the feedback control to correct the opening degree of the throttle valve.
Considering the above problems, the present invention aims at providing an intake air control scheme that implements a constant negative pressure control with a smaller number of process steps and with a higher accuracy under various environments.